Telomeres cap and protect the chromosome end by preventing genetic information from being lost or fused to other ends of chromosome by DNA repair pathway. Human telomeres comprise 50 to 200 TTAGGG repeat sequence of DNA that is coated and guarded by shelterin protein complex. Telomeric DNAs are shortened due to the end replication problem that sets the finite lifespan of cells. All cancer cells escape this limitaion by either up regulating telomerase, an enzyme that elongates telomeres or by activating an alternative lengthening of telomeres (ALT) pathway, resulting in immortal phenotype. Therefore, treatments that target the telomere itself would disrupt both mechanisms that cancer cells use to sustain unlimited proliferation. Hence, it is essential to understand the mechanisms of telomere regulation, extension, and maintenance. Despite the wealth of biochemical studies, there is to-date no study on the dynamics of the telomere. Completion of the proposed research will provide the dynamic axis of the how the telomere, telomere associated proteins, and telomerase orchestrate the protection and extension of the telomere. This proposal will use a novel single-molecule fluorescence system that will allow real time visualization of the protein- telomere dynamics and allow quantification of the extension rate of the telomere. We have established a real-time single molecule assay for probing the intermediates of the telomeric overhang and will extend the current method to visualize the telomerase extension. We aim to investigate how telomeric overhang conformation is related to telomerase activity and how telomere binding proteins can modulate overhang dynamics and telomere extension. Our preliminary data suggests that the telomere overhang has multiple structural conformations that exist in dynamic exchange. We hypothesize that telomere lengthening is promoted when the telomeric overhang is in a dynamic state and inhibited in a stably folded state. Understanding the mechanisms that regulate the chromosome end dynamics and accessibility can lead to identification of new targets for anti-cancer therapies, with increased specificity to telomeres. We propose that therapies that target the overhang structure, rather than telomerase itself, may hold great promise because they can inhibit both pathways that cancer cells use to proliferate indefinitely.